The goal of these studies is to increase the understanding of factors that regulate embryonic gene expression. It is obvious, that knowledge of the regulatory mechanisms that govern normal development are crucial for the understanding of abnormal development, resulting e.g. in congenital defects or in malignant growth. The developing tooth, and particularly the differentiating dental mesenchymal cells, are used as a model system. Hence, it is expected that these studies will increase the understanding of factors that regulate normal and abnormal tooth development. The lineage of dental mesenchymal cells originates in the neural crest, and after sequential determination and differentiation into specialized cell types, gives rise to all structures of the tooth and its supporting tissues, except the enamel. The first specific aim of this study is to analyse the molecular changes that accompany the sequential differentiation of the dental mesenchymal cells. Specific emphasis is given to syndecan, a recently characterized cell surface molecule, and tenascin, a matrix glycoprotein, which are expressed in the dental mesenchyme during early tooth morphogenesis, but also some growth factors and their receptors will be analysed. The expression of these different molecules is analysed by in situ hybridization and Northern blots, and their biological roles are studied in functional in vitro organ culture experiments. The second aim is to analyse the mechanisms that regulate gene expression in the differentiating dental mesenchymal cells. The roles of tissue interactions will be studied in experimental model systems involving heterotypic epithelial-mesenchymal explants and transfilter cultures. These cultures will be analysed by various histological techniques as well as in situ hybridization. Special emphasis is given to analysis of syndecan gene regulation both by in situ hybridization and filter hybridization techniques. The third aim is to analyse the biochemical properties of dental mesenchymal syndecan. Syndecan is a matrix receptor which has so far been characterized only in epithelial cells. In addition to biochemical and molecular biological characterization of syndecan its interaction with tenascin will be analysed. The hypothesis that an interaction between syndecan and tenascin is involved in dental mesenchymal cell condensation, will be tested. The fourth aim is to search for new developmentally regulated molecules in the dental mesenchyme. By using the hybridoma technology, and murine and bovine embryonic dental mesenchyme as immunogens, monoclonal antibodies have been produces, that recognize antigens in the dental mesenchyme, which appear to be developmentally regulated. The aim is to characterize these antigens and to analyse their significance for dental development. A cDNA library of dental mesenchymal cells will also be constructed for differential screening. This may result in discovery of developmentally significant genes associated with dental mesenchymal cell differentiation.